安卓：实现返回键“再按一次退出程序”

只需要在返回事件中写入少许代码即可 

@Override
 public boolean onKeyDown(int keyCode, KeyEvent event) {

 if(keyCode == KeyEvent.KEYCODE_BACK && event.getAction() ==
 KeyEvent.ACTION_DOWN){
 if((System.currentTimeMillis()-exitTime) > 2000){
 Toast.makeText(getApplicationContext(), "再按一次退出游戏",

Toast.LENGTH_SHORT).show();
 exitTime = System.currentTimeMillis();
 } else {
 finish();
 System.exit(0);
 }
 return true;
 }
 return super.onKeyDown(keyCode, event);
 }